In recent years, an electric power steering device (EPS) having a motor as a driving source has a need for continuing to drive the motor insomuch as safety is secured, even when a system has any anomaly. That is, even when the anomaly occurs, an increase in a steering force is suppressed to reduce a driver's burden, so that a vehicle is retreated more safely or is traveled to a facility (for example, dealer or repair shop) for removing the anomaly having occurred.
Thus, it is determined whether an electric conduction defect, which is caused by a break of a power feeding line or a contact failure (open anomaly of each switching element) of a drive circuit, for example, occurs in any one of respective phases (U, V, W) of the motor. When the electric conduction defect occurs in only one phase, the motor driving is continued while using the two phases except for the electric conduction defective phase as an electric conductive phase (for example, refer to Patent Documents 1 and 2).
That is, even when the electric conduction defect occurs in any one phase, the safe electric conduction can be made for the phases other than the electric conduction defective phase, like usual times. Hence, according to the above configuration, it is possible to continue the motor driving and to thus apply an assist force to a steering system while securing the safety.